Mechanical devices are used for cooling, heating, and thermal transfer in various applications. For example, mechanical coolers are used to cool certain sensor elements, to cool materials during semiconductor fabrication, and to cool superconductors such as in Magnetic Resonance Imaging (MRI) systems. Mechanical coolers typically utilize a thermodynamic cycle (often involving the compression and expansion of a fluid) to shift heat and create cold portions that are useful for cooling. Cryocoolers are a class of mechanical coolers that can achieve cold temperatures in the cryogenic range (e.g., <˜123 K). Different types of mechanical coolers may comprise various valves, thermal compressors, mechanical compressors, displacers, etc., to bring about expansion and compression of the working fluid.
Regenerative mechanical coolers operate by generating an oscillating pressure in a working fluid. Examples of regenerative coolers include Stirling coolers, Gifford-McMahon coolers and pulse tube coolers. Pulse tube coolers are advantageous in many applications because they do not include moving parts at the cold end, such as displacer pistons or valves. It is desirable, however, to create pulse tube coolers with increased efficiency and lower temperatures.
Pulse tube cryocoolers do not have moving parts at the cold end, such as displacer pistons or valves. To achieve the desired cooling, the combination of the phase control device and the reservoir cause a phase shift between mass waves and pressure waves generated by the compressor. By varying the mass flow to the buffer volume, the phase control device may serve to shift the phase of the mass flow relative to the pressure wave generated by the compressor.
Multistage pulse tube coolers are used to achieve temperatures colder than can be achieved with a single cooler alone. Multistage coolers can be arranged in series, where the first stage regenerator is connected to the hot end of the second stage regenerator, or in parallel, where the cold end of the first stage pulse tube is connected to the hot end of the second stage pulse tube. Some load shifting between stages can be brought about by varying the frequency, charge pressure and/or temperature of each stage.